Cooling mechanisms are commonly implemented to constrain the temperature of high-heat-flux devices within an allowable range. Thermal drift can result in various negative consequences, including decreased efficiency, reduced life, and even device failure. When a high-power device actuates in a transient manner, the challenge of maintaining stable temperature is considerably more difficult, especially if the thermal time constant of the device is relatively small. Indeed, the cooling must be cycled very rapidly to match the pulsed heat load.
For example, laser diode arrays require uniform temperature, both spatially and temporally, to reduce wavelength shift and spectral broadening. A lasing event is commonly implemented as a series of high current density quasi-continuous-wave pulses that are between 10 μs to 1 ms in duration and induce a local temperature change of 1-10° C. This short timescale thermal behavior is overlaid on a longer timescale response resulting from the average power (˜100 ms). The spectral output of an 808 nm GaAs quantum-well semiconductor laser shifts by 0.27 nm ° C.−1 as the mean temperature of the device drifts.
In this and similar applications, the objective is to keep the device at a near-constant temperature throughout the duration of device actuation. To achieve this, the cooling mechanism must be tuned to the thermal mass and heat output of the device. A higher initial rate of cooling would offset the thermal mass of the device, preventing thermal overshoot during the transient start-up period, while ongoing cooling could ensure stable temperature after the initial thermal transients die out.
Under such conditions, a cooling mechanism designed for transient performance can significantly improve device performance. Of the commonly employed cooling approaches capable of moderate-to-high heat flux, some are better suited to transient actuation than others. While pool/flow-boiling processes typically have long start-up periods that are not ideal, other methods such as jet-impingement, spray, and microchannel cooling are more promising candidates for transient performance. Nevertheless, none of these conventional cooling techniques rely on inherently transient phenomena—indeed, they assume steady state and are only initiated and then sustained continually. With conventional techniques, even if the cooling mechanism can be activated in a stepwise manner, the thermal resistance and capacitance of the system results in thermal fluctuations that cannot be entirely eliminated.
Conversely, flash boiling of a liquid pool is unique in that the phenomenon is inherently transient, with phase-change typically initiating at a maximum and decaying with time. In contrast, phase change in many conventional architectures is limited to a thermal boundary layer near the heated surface, characterized by the wall superheat.
The term “flash boiling” includes a number of different phase-change processes that initiate from a pressure drop, whether due to the fluid flowing through a pressure gradient or the depressurization of an otherwise stationary body of fluid. This rapid depressurization to a condition below the saturation pressure causes the vapor pressure of the liquid to meet or exceed the pressure of its environment and results in a body of liquid that is uniformly superheated. The liquid superheat, which is not easily attainable in traditional boiling hierarchies and is typically characterized by the Jakob number, provides the requisite latent enthalpy for rapid phase change and, as a result, high rates of expansion and cooling.
The flash boiling of a stream of fluid, i.e., a flashing flow, is commonly used for atomization processes in which a liquid is injected into an environment where the pressure is at or below the vapor pressure of either the fluid or one of its constituents. Typically, upon injection the flashing fluid is in a significantly superheated state, resulting in a rapid phase-change process and an associated fine dispersion. This technique is considered to be one of the more effective methods of atomizing a fluid and its use ranges from domestic aerosols to direct fuel injection.
The alternate scenario, in which a body of fluid is depressurized, has been studied at widely varying degrees of nonequilibrium ranging from highly restricted venting of pressurized vessels to explosive homogenous nucleation at the superheat limit. As it does with flashing flows, the rate and the magnitude of the pressure drop significantly alters the nature of the phase-change process experienced by the fluid. If the rate of depressurization is sufficiently low, the liquid-gas system can be considered to remain in thermodynamic equilibrium throughout the entire event, and the phase change can be treated as a thermodynamically limited process. More commonly however, a certain degree of superheat will be attained by the liquid as controlled by the kinetics of the phase-change process.
Once the fluid attains a certain level of superheat, heterogeneous boiling may initiate depending on the size and density of nucleation sites available and, in transient processes, the amount of time the fluid dwells at the given level of superheat. Higher levels of superheat will typically result in a faster heterogeneous boiling process as more nucleations sites become active. These nucleation sites can include surface defects that are not fully wetted and absorbed gas within the liquid. A liquid can remain in this metastable state for a practically indefinite amount of time if no sufficient sites are available.
If the superheat is attained quickly enough as determined by the nucleation sites available, the fluid can reach the superheat limit and initiate homogeneous boiling. At this limit, the fluid reaches a point of thermodynamic instability, and boiling occurs volumetrically throughout the fluid irrespective of the presence of nucleation sites.
Flash boiling of a pool of liquid can also occur at intermediate levels of superheat. Upon depressurization, the phase-change process commonly is manifested as a boiling front that propagates from the liquid-gas interface downwards through the liquid. Alternately, the boiling event can initiate and propagate from the surfaces of the vessel.
Despite the finite duration of the event, flash-boiling may still be suitable for transient applications that require pulsed cooling for time periods in the range of 100 milliseconds to 10 seconds. In particular, the flash results in an initial peak cooling that wanes to a quasi-steady state value, which may be useful for counteracting the thermal mass of a heated device to achieve a more constant transient temperature. What is needed are heat management methods that apply flash boiling to heat-heat-flux cooling, as well as convective cooling systems that utilize flash boiling—or similar transient phenomena—to efficiently transfer heat from a hot body via a phase change and/or conduction.